Multiple-section extension ladders conventionally use a halyard, in combination with a pulley and a rung engaging lock, to raise and lock the extended section to the required height. Previously, many types of locks have been used in endeavoring to provide an effective means to hold the section in its extended position. These locks are normally effective. However, there have been incidents, especially in hurried emergency situations, where the lock has failed causing the extended section to slip and cause bodily harm.
A search of the prior art did not disclose any patents that read directly on the claims of the instant invention, however, the following U.S. patents were considered related:
______________________________________ U.S. Pat. No. Inventor Issue Date ______________________________________ 4,299,306 Hawkins 10 November 1981 2,310,441 Klum 9 February 1943 2,210,803 Dunn 6 August 1940 1,964,067 Leach et al 26 June 1934 304,420 Fox et al 6 June 1884 750,504 Tiefel 13 May 1903 687,945 Waggner 18 March 1901 613,848 Seagrave 29 August 1898 ______________________________________
Hawkins teaches a lock having a guideway arm attaching to two rungs simultaneously with a second member guiding the lock past the station of a ladder when the ladder is being extended or retracted.
Klum utilizes a lock having two triangular shaped members with an opening therethrough rigidly attached at their apexes to a rung, the ends of which are rotatably received in sockets secured to the legs of the ladder. A rope is attached over a sheave on a tie rod on one end and to the rung on the other, looping through a second sheave above the rung. The operator uses one portion of the looped rope to disengage the hook on the lock and the other end for hoisting.
Dunn practices a lock utilizing a bar formed into a loop around a rung with a hook on the opposite end. This bar is spring loaded and a cable is looped around a pulley for hoisting. In operation, tensioning the cable retracts the bar against the action of the spring allowing extension or retraction. To stop the ladder, the cable is released, suddenly extending the hook by spring pressure allowing the hooked end of the bar to be in contact with the next rung therebelow.
Leach et al employ a hook on the end of an arm that is connected to a rotatable rung. The rung also has a pulley attached to a bracket. When pressure is exerted on a rope throug the pulley, the angle of the arm is such that it pulls away from the rung upon which it is resting, allowing the ladder to be extended. When pressure is released, the hook is rotated inward by gravity locking into place on the next available rung.
Fox et al use a sheave disposed centrally on a rail over which a cord passes connected to an arm of a rock-shaft journaled on a rotatable shaft. A pair of upwardly projecting hooks are secured near the ends of the shaft and rotate as the rope is pulled disengaging the hooks when tension is applied. Release for retraction is accomplished in the reverse procedure.
For background purposes and as indicative of the art to which the invention relates reference may be made to the remaining cited patents.